Field of the Invention
This invention relates to color cathode ray picture tubes used in television sets and computer monitors. It is addressed specifically to means and methods for facilitating the manufacture of tubes that utilize the tension foil shadow mask.
The image that appears on the faceplate of a tension mask color cathode ray tube results from the excitation of phosphors deposited on the inner surface of the faceplate by three electron beams emitted by an electron gun in the neck of the tube. Three different phosphors are deposited on the inner surface that emit red, green and blue light when excited. Selective excitation of the respective phosphors is accomplished by the imposition of a tensed, perforated foil known as the "shadow mask" between the source of the three beams and the phosphor screen. The shadow mask acts as a parallax barrier which ensures that a respective beam will excite only the intended phosphor targets.
The phosphors are deposited on the screen area of the inner surface of the faceplate in a predetermined pattern which consist of triads of red, green and blue dots of phosphor to form what is termed a dot screen.
The dots of phosphor that make up each triad are enclosed by a dark material known as the black surround. In the screen manufacturing process, the black surround is deposited first, forming a black pattern on the inner surface of the faceplate having round, transparent apertures for receiving respective ones of the three color phosphors. The green-light-emitting phosphor is then deposited, forming a green dot in the aperture in each triad allotted to receive a green phosphor dot. The phosphors that emit blue and red light are next deposited in sequence in their allotted spaces in each triad.
The apertures in the tensed foil shadow mask are used to photolithographically form the screen, using light sources that simulate the location of each of the three beams emitted by the electron gun.
The process of depositing a screen is a process of great precision in view of the fact that, in a 14-inch (diagonal measure) high-resolution tension mask tube, there are about 2.5 million phosphor dots, formed by 830,000 apertures in the shadow mask.
The materials of the black surround and the three phosphors are typically applied to the screening surface of the tube in the form of screening fluids known as as slurries. The slurries comprise suspensions of phosphor particles in an actinic medium that will dry by evaporation, such as polyvinyl alcohol. The slurries are applied to the screening area by spin application; that is, the slurry is poured onto the screening area of the faceplate as the faceplate is rotated. The faceplate is then inverted, and the excess slurry poured off.
In the process of screening, it is imperative that no remnant of slurry be left in the vicinity of the faceplate. Particles can flake off and occlude one or more of the apertures of the closely adjacent shadow mask, resulting in rejection of the finished tube. Also, particles can become electrically charged and circulate within the tube envelope, causing electrical shorts. Remnants of slurry are especially prone to accumulate beneath the tensed foil shadow mask attachment caps that may overhang the ceramic structures of mask support structures, such as the structures shown and described in U.S. Pat. No. 4,891,546 of common ownership herewith.
In commonly owned U.S. Pat. No. 5,053,674, there is disclosed a generally rectangular mounting structure for attaching a tensioned foil shadow mask to a flat glass faceplate of a color cathode ray tube. The structure comprises four rails attached at respective ends thereof by means of four corner brackets spot-welded to adjacent ends of a pair of rails. Each of the four corners is coated with a solder-glass-based sealant to hermetically seal the corner bracket-rail interface. The solder glass is devitrified by baking. The resultant seal prevents phosphor particles from collecting in the gaps between the corner brackets and the coupled rails.
In U.S. Pat. No. 4,804,881 to Strauss, also of common ownership herewith, a tension mask support structure is disclosed in which the structure is sealed to prevent any outgassing from the interior of the structure during the life of the tube. The hollow mask support elements are joined at the corners by connectors which have necked-down portions that plug into the elements. The corner connectors are then welded and brazed to the elements to completely seal off the structure.